1. Field of the Invention
The present invention relates to an organic electroluminescent device, more particularly, to a dual panel type organic electroluminescent device and a method of fabrication thereof.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic electroluminescent devices (ELD) have been of particular interest in research and development because they are light-emitting type displays that feature wide viewing angle and desirable contrast ratio, as compared with liquid crystal display (LCD) devices. Because such organic ELDs do not require a backlight, they are small and lightweight, as compared to other types of display devices. The organic ELDs have other desirable characteristics, such as low power consumption, superior brightness and fast response time. When driving the organic ELDs, only a low direct current (DC) voltage is required. Moreover, a fast response speed can be obtained. It is understood in the industry that because the organic ELDs are entirely formed with solid materials, which is different from LCD devices, they are sufficiently strong to withstand external impacts and also have a wider operational temperature range. Moreover, because fabricating organic ELDs is a relatively simple process with a few processing steps, it is much cheaper to produce organic ELDs than LCD devices or plasma display panels (PDPs). In particular, manufacturing the organic ELDs only requires deposition and encapsulation apparatuses.
An active matrix organic ELD has a storage capacitor in each pixel to maintain a voltage that is applied to the pixel until the next frame, regardless of the number of the scanning lines. Because a uniform brightness is obtained throughout the pixels with a low applied current due to the storage capacitor, an active matrix organic ELD of low power consumption, high resolution and large area can be manufactured.
FIG. 1 is a schematic cross-sectional view of an organic ELD according to a related art. Referring to FIG. 1, first and second substrates 10 and 20 face each other and are spaced apart from each other. An array element layer “AL” is formed on the first substrate 10, and includes a plurality of thin film transistors (TFTs) “T.” Although not shown in FIG. 1, the array element layer “AL” further includes a plurality of gate lines and a plurality of data lines crossing the plurality of the gate lines. A pixel region “P” is defined at a crossing of the gate and date lines. The TFTs “T” are located in the pixel region “P.” In addition, a plurality of first electrodes 22, an organic electroluminescent (EL) layer 24 and a second electrode layer 26 are sequentially formed on the array element layer “AL.” The first electrodes 22 are connected to the TFTs “T.” The first electrode 22, the organic electroluminescent (EL) layer 24 and the second electrode layer 26 constitute an organic EL diode “DEL” in the pixel region “P.”
Meanwhile, the second substrate 20 is an encapsulating panel having a receded portion “RP.” The receded portion “RP” has an absorbent 28 in order to protect the organic ELD from moisture. A seal pattern 29 is formed between the first and second substrates 10 and 20, specifically, at an outermost edge thereof. With the seal pattern 29, the first and second substrates 10 and 20 are attached to each other.
FIG. 2A is a schematic plan view of the organic electroluminescent device according to the related art. In FIG. 2A, a gate line 37 crosses a data line 51 and a power line 42, with the data line 51 and the power line 42 being spaced apart from each other. A pixel region “P” is defined by the gate line 37, the data line 51 and the power line 42. A switching thin film transistor (TFT) “TS” is located adjacent to the crossing of the gate line 37 and the data line 51. A driving TFT “TD” is connected to the switching TFT “TS” and the power line 42. A storage capacitor “CST” uses a portion of the power line 42 as a first capacitor electrode and an active pattern 34 extending from an active layer 31 of the switching TFT “TS” as a second capacitor electrode. A first electrode 58 is connected to the driving TFT “TD,” and an organic electroluminescent (EL) layer (not shown) and a second electrode (not shown) are sequentially formed on the first electrode 58. The first and second electrodes and the organic EL layer interposed therebetween constitute an organic EL diode “DEL.”
FIG. 2B is a schematic cross-sectional view taken along the line “II-II” in FIG. 2A. In FIG. 2B, the driving thin film transistor (TFT) “TD” including an active layer 32, a gate electrode 38 and source and drain electrodes 50 and 52 are formed on a substrate 10. The source electrode 50 is connected to the power line 42, and the drain electrode 52 is connected to the first electrode 58. An active pattern 34 made of the same material as the active layer 32 is formed under the power line 42 with an insulating layer 40 interposed therebetween. The active pattern 34 and the power line 42 constitute the storage capacitor “CST.” An organic electroluminescent (EL) layer 64 and a second electrode layer 66 are sequentially formed on the first electrode 58, and constitute an organic EL diode “DEL.” A first insulating layer 30 is formed between the substrate 1 and the active layer 32. The first insulating layer 30 may serve as a buffer layer. A second insulating layer 36 is formed between the active layer 32 and the gate electrode 38. A third insulating layer 40 is formed between the active pattern 34 and the power line 42. A fourth insulating layer 44 is formed between the power line 42 and the source electrode 50. A fifth insulating layer 54 is formed between the drain electrode 52 and the first electrode 58. A sixth insulating layer 60 is formed between the first electrode 58 and the organic EL layer 64. The third to sixth insulating layers 40, 44, 54 and 60 include contact holes for electric connections.
In the organic ELD according to the related art, the array element layer having TFTs and the organic electroluminescent (EL) diode are formed on the first substrate, and the second substrate is attached to the first substrate for encapsulation. However, when the array element layer having TFTs and the organic EL diode are formed on one substrate in this way, production yield of the organic ELD is determined by a multiplication of the array element layer's yield and the organic EL diode's yield. Because the yield of the organic EL diode is relatively low, the production yield of the overall ELD becomes limited by the yield of the organic EL diode. For example, even when TFTs are well fabricated, an organic ELD using a thin film of about 1000 Å thickness can be determined to be defective due to a defect of an organic emission layer. This results in loss of materials and high production costs.
In the meanwhile, organic ELDs are classified into a bottom emission type and a top emission type according to an emission direction of light used for displaying images via the organic ELDs. The bottom emission type organic ELDs have such advantages as high encapsulation stability and high process flexibility. However, the bottom emission type organic ELDs are ineffective for high resolution devices because they have a low aperture ratio. In contrast, the top emission organic ELDs have a higher expected life span because they are more easily designed and have a high aperture ratio. However, in the top emission type organic ELDs, the cathode is generally formed on an organic emission layer. As a result, transmittance and optical efficiency of the top emission type organic ELDs are reduced because of a limited number of materials that can be selected. When a thin film passivation layer is formed to avoid the reduction of light transmittance, the thin film passivation layer may fail to block infiltration of exterior air into the device.